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We present a novel method for the measurement of lithium isotopes in garnet utilising glass reference materials and secondary ion mass spectrometry (SIMS). Measured lithium isotopic compositions of natural garnets are heterogeneous, making them unreliable reference materials forin situdetermination. However, SIMS lithium isotope measurements of glasses derived from these natural garnets are isotopically identical to their parent garnets and more homogeneous, demonstrating that they can be used as reliable reference materials. To characterise the composition dependence of instrumental mass fractionation (IMF), oxide and silicate powders were used to synthesise custom‐made glass reference materials (CGRMs) with garnet‐equivalent compositions. Results for six CGRMs measured by SIMS show a significant linear relationship between IMF and FeO and MnO contents. Corrections for this compositional IMF result in changes of up to 12‰ within the compositional range explored. Uncertainty in IMF‐corrected SIMS analyses with a 20 μm spot is in the range of 2.5 to 4.5‰, depending on the garnet composition and reference materials used. The method forin situlithium isotope measurement in garnet by SIMS presented here is highly adaptable, valid across a range of Al‐rich garnet compositions, and yields spatial resolution and precision necessary to address a range of geological applications.

Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ¹¹B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ¹¹B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO₂and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, thein situboron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).